1. Field of the Invention
The present invention relates to a method of recording data on a rewritable optical disk or more specifically, a phase-change optical disk, and to an apparatus for recording and reproducing the data.
2. Description of the Prior Art
One of recording mediums capable of recording information is known as an optical disk. Namely, phase-change optical disks and opto-magnetic disks which are capable of rewriting data at a higher speed and storing huge amounts of information are widely utilized as video editing apparatuses or storage apparatuses of microcomputers.
Principles of recording on and reproducing from a phase-change optical disk will be explained. The phase-change optical disk has a recording layer made of a metal film e.g. GeTeSb. Recording is made by directing a laser beam intensified with an optical head to a recording track on the recording layer of the disk. When the laser beam is high (as a recording power), it heats up the recording layer to a melting temperature. Upon irradiation of the laser beam being stopped, melted regions of the recording layer are rapidly cooled down and turn to an amorphous phase. When the laser beam is not so high (as an erasing power), it heats up the recording layer to a crystallizing temperature thus turning irradiated spots to a crystallized phase. By irradiating on a rotating disk alternately two, recording and erasing, power modes of the laser beam modulated according to data to be recorded, irradiated spots with the recording power are turned to amorphous phases or recorded regions (marks) and also, irradiated spots with the erasing power are turned to crystallized phases or non-recorded regions (spaces). The phase change can be made from the crystallized phase to the amorphous phase or vice versa. Hence, it is possible to erase any mark on the recording layer and to perform direct overwriting. In overwriting, it is essential for saving data at high accuracy to provide a high degree of erasability which represents a ratio of erased and not-erased portions. For reproducing such phase-change recorded data, a reproducing power of laser beam which is too low to cause a phase change is used. When the reproducing power of laser beam is irradiated, its reflection is large on crystallized spots but small on amorphous spots so that it can read recoded data.
A mark edge recording method is one of most common high-density recording techniques, in which each mark and space represents a train of 1s or 0s and data is assigned to both lengthwise edges at front and rear of the mark. For detecting the mark edge at high preciseness during reproduction, it is substantial in recording to have the mark structured uniformly not only in length but also in width. Accordingly, a shape of the mark is one of fundamental factors for high-density recording.
A uniform, correct size of the marks in the mark edge recording method may be built by using a multi-pulse laser beam. As shown in FIG. 2(a), if the recording and erasing power modes of laser beam are simply used for recording of data, a resultant mark is widened at its rear part of the mark due to heat storage effects on the recording layer of a disk, thus having a tear-drop shape. The tear-drop shape causes the rear detection edge to be rearwardly dislocated and may thus create a more number of crosstalk. For compensation, a waveform of the recording power of the laser beam is modified to a combtooth shape by application of multiple pulses, as shown in FIG. 2(b). Accordingly, as a heating to the rear part of the mark is attenuated, the shape of the mark will substantially be uniform in width (see an example of such multi-pulse recording methods depicted in Japanese Patent Application 5-279513 (1993)).
The application 5-279513 explains development of marks with a constant width by controlling the recording power of the laser beam delicately with a recording signal of which waveform has been modified to the combtooth shape. However, any variation in a duty ratio of given pulses or the recording power of the laser beam will affect on the shape of the mark more or less. For example, if the recording power of the laser beam is lowered, the mark may be thinned at its rear part failing to carry data. If the recording power is increased, the rear part of the mark may be enlarged in width. It is requested for forming a uniform, correct size of the marks to carry out precise controlling actions over the recording power of laser beam and a duration of each pulse. More specifically, the above-explained multi-pulse method allows no generous margin of the recording power of the laser beam for error-free assignment of records.
There are proposed some methods of recording at two steps for increasing the erasability in overwriting. An example is disclosed in Japanese Patent Laid-open Publication 1-184631 (1989) in which a first action of the laser irradiation erases the amorphous spots and a second action of the same performs a known recording process to produce a series of amorphous and crystallized spots. It is intended to increase the erasability by repeating the erasing action. A another example depicted in Japanese Patent Laid-open Publication 3-141028 (1991) assigns the amorphous phase to a non-recorded mode (at erased state) and the crystallized phase to a recoded mode, thus ensuring improvement of the erasability as a recorded mode has been turned to a molten state which represents an erased state with the amorphous phase.
Furthermore, Japanese Patent Laid-open Publication 62-270024 (1987) discloses a method in which two time different laser beams, one delayed from the other, are used for crystallization of a specific recording layer which is hardly be crystallized by such a single laser beam. More specifically, a first laser beam is irradiated for heating to an amorphous mode temperature, and after a predetermined time, a second laser beam is applied for addition of supplementary heat so that crystallization is effected.
The methods described in the Japanese Patent Laid-open Publications 1-184631 and 3-141028 may be effective for improving the erasability but fail to have the marks of the mark edge recording produced identical in width. Also, the method depicted in the Japanese Patent Laid-open Publication 62-270024 has no teaching of having the marks arranged uniform in width with any of following cases. It is true that the two laser beams are eligible with a phase-change optical disk which is hardly crystallized by application of a single laser beam because a time difference is given between the two laser beams, and the time difference may arbitrarily be determined when recoding on a phase-change optical disk which is not difficult to be crystallized by a single laser beam. However, the Japanese Patent Laid-open Publications 1-184631, 3-141028, and 62-270024 do not disclose a solution which controls the mark identical in width.